The Problems of Deep Water Oil and Gas Drilling
The techniques of drilling for subsea reserves of oil and gas from floating vessels, as well as from non-floating offshore platforms, are well known. In the context of this invention, drilling operations performed on a non-floating platform erected on the ocean floor are analogous to oil and gas drilling operations performed on land, and such operations do not present the problem with which this invention is concerned. The problem to which this invention is addressed is currently encountered in the context of subsea drilling operations performed from floating structures; the floating structures may be either drillships of shipform nature or that class of floating structure known as semi-submersible drilling platforms. The problem is presented by the riser pipe which is connected between the subsea wellhead and the floating drilling platform to provide a return path for drilling mud to the platform from the subsea well.
In oil and gas drilling operations, whether performed on land or at sea, a water-based mineral slurry of controllable density is used to cause the hydrostatic pressure in the well being drilled to approximate, as closely as possible, the probable pressure of gas or oil in the subterranean formation being drilled. This slurry is called "drilling mud". By regulation of the density of the drilling mud in combination with the height of the column of the drilling mud in the well being drilled, a measure of control is obtained over the well to prevent blowouts. Blowouts can occur when the drill bit breaks into a formation where oil or gas exists under pressure. If no hydrostatic safety mechanism were provided in the well bore, emergence of the drill bit into a pressurized subterranean formation would result in the oil or gas in the formation "blowing out" through the bore and either destroying or presenting substantial hazard to the drilling platform and its personnel. Accordingly, drilling mud is used in connection with mechanisms known as blowout preventers either to prevent the occurrence of a blowout or to slow occurrence of a blowout until the blowout preventer mechanisms associated with the wellhead, either on land or at the ocean floor, can be operated to shut in the well and prevent the emergence of the highly pressurized oil or gas from the well bore.
Drilling mud is expensive. It is desirable that the drilling mud be returned from the well bore to the drilling platform so that it can be cleaned, processed and reused. Also, the contents of the drilling mud, as returned to the drilling platform, are informative about the formation being drilled at the time. The contents of the drilling mud, as analyzed at the drilling platform, can, among other things, give warning that a high pressure oil or gas pocket is being approached by the drill bit, and that the density of the drilling mud should be adjusted or other steps taken to control the well and prevent a blowout. Thus, from considerations of both economics and safety, it is desirable that the drilling mud be returned to the drilling platform.
In current typical offshore drilling operations, drilling mud is returned from the well bore to the drilling platform through a pipe extending from the submerged wellhead blowout preventer to the drilling platform substantially concentrically about the drill string. This pipe is known as the "riser pipe", or "riser", and is of large diameter relative to the drill string. Because of its large diameter, the riser is subjected to high levels of bending stress in response to lateral loading by ocean currents and the like. The riser pipe is made with relatively thick walls to enable it to withstand these bending stresses; this causes the riser assembly to be heavy. In order to maintain substantial concentricity of the riser relative to the drill string and to prevent the riser from buckling under its own weight, the riser is maintained under tension along its length by a tensioning mechanism located, usually, at the drilling platform. Typically, the riser tensioning mechanism is provided by a plurality of wire rope cables connected between the upper end of the riser and the platform. If the platform is a floating platform, constant tension devices are needed in this connection because the drilling platform moves vertically, as well as laterally, in response to wave and tidal action. Also, because of the presence of vertical motion of the platform relative to the upper end of the riser, current practice provides a telescoping slip joint in the riser. Usually the slip joint is at the upper end of the riser, and the duct for flow of the drilling mud from the riser to the drilling mud processing facility is connected to the riser via this slip joint.
For many years, the maximum depth of water in which offshore wells could be drilled from floating platforms was on the order of 300 feet due to the limitations of mooring systems for the floating platforms. This depth limitation has been expanded by improvements in mooring systems and by the development of dynamically positioned floating drilling platforms which require no mooring to the ocean floor. The rule-of-thumb maximum water depth in which offshore wells now may be drilled is on the order of 1000 feet, and this limitation is set by factors attendant to the riser. Conventional riser systems and riser tensioning arrangements can be used, albeit with some difficulty, in water depths up to, but not much beyond, about 1000 feet. One thousand feet appears to be a practical limit beyond which strengthened riser pipes and riser tensioning mechanisms of conventional configuration cannot be used economically. As a result, considerable thought has been applied in the offshore drilling industry to the development of wholly new riser systems and of riserless drilling systems.
It is useful to note that the bending moments which are developed in a beam are proportional to the cube (third power) of the length of the beam; this relation is modified somewhat if the beam is placed in tension along its length. Forces applied to a riser by ocean currents act on the riser in much the same way as vertical loads act on a horizontal beam. Thus, assuming constant loading conditions on risers of the same transverse size, a 600-foot riser would deflect roughly eight times as much as a 300-foot riser and would develop eight times the stress. However, it is desirable to keep the riser concentric to the drill string and to keep the drill string straight. Therefore, as risers become longer, it is necessary to make them more resistant to lateral deflection, and this in turn results in heavier and larger diameter risers which are more and more difficult to store, assemble, handle and adequately tension. Axial tensioning of risers is another problem separate from lateral bending.
Some deep water riser systems now in use create tension in the risers by attaching buoyant members to the outer circumference of the riser at various locations along the vertical extent of the riser; this technique has been used in risers up to 3000 feet long. A practical difficulty with this approach is that the buoyant members enlarge the silhouette of the already large riser and render the riser substantially more susceptible to lateral loads due to drage forces created by ocean currents. Also, buoyancy members are bulky and are difficult to store aboard the drilling platform prior to installation of the riser. Buoyancy members are difficult to connect to the riser, and their presence on the riser considerably complicates installation of the riser between the submerged wellhead and the drilling platform. Perhaps most significantly, buoyancy members are very expensive; their cost ranges from 5 to 10 Dollars per pound of buoyancy. Riserless drilling systems are subject to criticism because they may not provide a return path for the drilling mud to the drilling platform, thereby creating problems of economics and safety, as shown above.
Wire rope riser tensioning systems require periodic replacement of the riser tensioning cables. Safety considerations require that a wire rope cable be replaced after it has been used a specified time under specified load conditions. The greater the load to which the wire rope is subjected, and/or the more times it cycles back and forth across its supporting sheaves, the more frequently the wire rope should be replaced. As wire rope riser tensioning systems are subjected to greater and greater loads due to their association with larger and longer risers, the frequency of riser cable replacement increases. In a typical wire rope riser tensioning system, it may take 6 to 8 hours to renew the cables; this operation is potentially hazardous and often requires that the drilling operations be suspended as this operation is being carried out. The daily charter rate for an offshore drilling rig is quite substantial, and interruptions in drilling operations are to be avoided wherever possible.
It is therefore seen that, for efficient and economic support of offshore drilling operations in deeper and deeper waters, a need exists for substantial improvements in the handling and tensioning arrangements for risers of very heavy but rather conventional design.